1. Field of the invention
The present invention relates to an external magnetic field device for a magnetooptical information recording and reproducing apparatus in which information is recorded by modulating a magnetic field.
2. Description of the prior art
In a rewritable optical information recording system, a magnetooptical information recording apparatus is used to invert the direction of the magnetization of a magnetooptical information recording medium (hereinafter, referred to as merely "a medium") which, in the initial state, has been uniformly magnetized in either direction (the initial state). This magnetooptical information recording apparatus is operated by one of two methods: the optical modulation method; or the magnetic field modulation method.
When the optical modulation method is employed, the magnetization of a medium is inverted only when a laser beam irradiated thereon is enhanced in intensity. When information is rewritten, hence, the optical modulation method requires an erasing operation prior to the writing operation, so that the magnetization of the medium is returned to its initial state, with the result that the access rate is substantially decreased.
As a result, the magnetic field modulation method has begun to attract attention. In the magnetic field modulation method, the polarity of an external magnetic field which is generated by an external magnetic field device and applied to a medium is changed to invert the magnetization of the medium, while the intensity of the laser beam is kept constant. Therefore, the magnetization of the medium always corresponds to the direction of the external magnetic field, enabling a direct overwrite in the rewriting operation (i.e., information can be rewritten without executing the erasing operation).
However, the magnetic field modulation method has the following two drawbacks.
When information is recorded, an alternate current must be continuously supplied to an external magnetic field device, thereby increasing the copper loss and iron loss of the device. This causes the external magnetic field device to generate a large amount of heat which should be dissipated therefrom. Conventionally, the sole measure for solving this problem is to install a cooling fan for cooling the external magnet. The installation of a cooling fan in the device causes other problems in that the production cost of the apparatus is increased, that a noise is generated from the cooling fan, and that a large space for installing the cooling fan is required.
The second problem is that the recording of information requires an external magnetic field of hundreds Oe (about 1.about.5.times.10.sup.4 A/m). When a magnetic coil is employed in an external magnetic field device, therefore, the number of turns must be great, resulting in a high inductance of the coil. This high inductance of the coil causes the magnetization reversing time to be long, thereby increasing the difficulty in the recording in the order of MHz.
If a medium in which information can be recorded under a weak magnetic field is used, the number of turns of the coil can be reduced so that the use of such a medium may eliminate the above-mentioned problem. In this case, however, it is necessary to consider a leakage magnetic field generated by an actuator for focusing or tracking the laser beam in the apparatus because such a medium is influenced by a weak magnetic field.